Optical coherence tomography (OCT) is an optical method, by means of which 3D images of optically scattering media such as, for example, biological tissue can be recorded with a resolution in the micrometer range. In this case, the depth direction of the sample, i.e. the dimension along the irradiating direction of a sample beam, is captured in that—in simplified terms—the distance of scattering centers within the sample from the light source is measured by means of interferometric signals. Light with a comparatively long wavelength, typically in the near-infrared range, is used in OCT. This has the advantage that the light can penetrate comparatively deep into the scattering medium such that its scans sections can be recorded with near microscopic resolution. OCT has been successfully used, in particular, in the imaging of the human eye, in which high resolution images of the retina and the front eye section can be recorded. Another very attractive application of OCT, which is currently the object of intensive development activities, concerns intravascular imaging in the field of cardiology.
As in all interferometric methods, OCT too makes it possible to measure the interference signal in the time domain or in the frequency domain. The first process is referred to as TD(Time Domain)-OCT and the second process is referred to as FD(Frequency Domain)-OCT.
A particularly efficient variant of FD-OCT is so-called “swept-source OCT” that is explained with reference to FIG. 1. FIG. 1 shows a swept-source OCT system 10 with a tunable light source 12, namely the so-called “swept-source” 12. The tunable light source 12 carries out wavelength sweeps with a certain repetition rate fsweep, wherein the tunable light source generates in these wavelength sweeps optical signals, in the time history of which the wavelength of the signal changes. Signals of this type are also referred to as “chirps.” The signal is coupled into an interferometer 14 that has a first arm 16 and a second arm 18. In the first arm 16, the optical signal is delivered to a sample 22 via a circulator 20. The optical signal penetrates into the sample 22 up to a certain depth and is scattered in the sample 22 such that a portion of the light is reflected back into the first interferometer arm 16. Depending on the local composition of the sample 22, the optical signal is respectively scattered or reflected to a greater or lesser degree. In this way, layers underneath the surface of the sample 22, on which the optical signal is respectively scattered or reflected comparatively strong, can be rendered “visible.”
The second arm 18 of the interferometer 14 is a reference arm that contains a delay loop 24. The optical signals in the two interferometer arms 16 and 18 are superimposed, for example, with the aid of a 50/50 coupler 25 and an interference signal 34, which is also referred to as a “fringe signal” 34 in the pertinent field, is generated, for example, with the aid of photodetectors 26 and a differential amplifier 28.
The interference signal 34 therefore consists of the superposition of two optical signals with time-variant wavelengths (chirps). In this case, the two superimposed signals are shifted in time to a degree that is dependent on the transit time differential along the first and the second arm 16, 18. This transit time differential in turn depends on the depth, at which the light is scattered within the sample 22. In an exemplary embodiment, the optical wavelength differential and therefore the time shift of the signals relative to one another decreases as the depth of the scattering center increases. Since the wavelength of the signals is time-variant due to the wavelength sweep, the wavelengths increasingly deviate from one another in the superposition as the time shift increases such that a beat is generated in the interference signal, wherein the frequency of this beat rises as the time shift increases. In simple terms, the respective scattering or reflection on a low-lying scattering center of the sample 22 results in a high-frequency interference signal 34 in this case and the scattering on the surface of the sample 22 results in an interference signal 34 with low beat frequency. Depending on the mutual adaptation of the lengths of the interferometer arms 16, 18, however, a small transit time differential (and therefore the low beat frequency) can also be obtained for a low layer rather than for the surface. Regardless of the special configuration, depth scans of the sample 22 can in this way be generally recorded in the form of frequency information.
FIG. 2 shows the schematic design of a conventional OCT system, in which the interferometer 14 is designed similar to FIG. 1, but not illustrated in detail. The OCT system 10 according to FIG. 2 likewise contains a tunable light source 12 that receives a sweep control signal 30 generated by an oscillator 32. The sweep control signal 30 defines the repetition rate fsweep, with which the light source 12 is tuned. For this purpose, the sweep control signal 30 itself may have the frequency fsweep or a multiple thereof.
It should be noted that the oscillator 32 may form part of the light source unit 31 that is schematically illustrated in the form of a box drawn with broken lines in FIG. 2. For example, the oscillator 32 may form part of the control electronics of the light source unit 31. The sweep control signal 30 may consist of any suitable signal, the frequency of which influences the repetition rate fsweep of the light source 12. The concrete configuration of the sweep control signal 30 depends on the respective design of the tunable light source 12 in this case. FIG. 2 furthermore shows that a feedback 32 between the tunable light source 12 and the oscillator 32 may be provided in order to adapt the frequency of the oscillator 32 to the operation of the tunable light source 12. This is important, for example, if the light source 12 used consists of an FDML laser, in which the sweep rate needs to be precisely adapted to the cycle time of the light that may vary, for example, due to temperature fluctuations.
The interference signal 34 from the interferometer 14 is sampled with an AD converter 36 that represents a detection device for the time-resolved detection of the interference signal 34. In the context of the present invention, the interference signal 34 is detected “with the aid of” the detection clock signal 38. In other words, the detection clock signal 38 is used in the operation of the detection device 36 and influences or directly or indirectly defines the time response of the detection device 36. It may particularly define the points in time or the cycle, in which the interference signal 34 is detected or sampled. In the concrete exemplary embodiment, the AD converter 36 has an input for a detection clock signal 38 that is generated by an additional oscillator 40. The AD converter 36 converts the interference signal 34 into digital data 42 at points in time that are defined by the detection clock signal 38. In the illustration in FIG. 2, the AD converter 36 and the additional oscillator 40 form part of a data acquisition unit 41 that is illustrated in the form of a box drawn with broken lines. The nominal frequencies of the oscillators 32 and 40 have a certain, not necessarily integral relation to one another such that the time history of the frequency sweep and of the detection can be computationally correlated, but the oscillators 32 and 40 in principle operate independently of one another. In the context of the present disclosure, the term “oscillator” should be interpreted in a broad sense and is not e.g. limited to any special type of oscillator such as, for example, a quartz oscillator. The oscillator may also consist, for example, of a DDS, an analog oscillator that is controlled to a resonance of the tunable light source, etc.
Each wavelength sweep of the light source 12 corresponds to a depth scan of the OCT system 10. At the beginning of each wavelength sweep, a trigger signal (not illustrated in FIG. 2) is transmitted to the AD converter 36 in order to assign the digital data 42 to the associated wavelength sweep.
The detection clock signal 38 has a frequency fsamp that defines the sampling rate, with which the interference signal 34 is sampled by the AD converter 36. In this case, the sampling rate fsamp typically amounts to at least 2.5-times the highest frequency of the interference signal 34 to be detected. According to the Nyquist criterion, the double sampling rate would suffice, but the response at the upper end of the frequency band decreases in this case, wherein this decreased response is in practical applications prevented with increased oversampling by factors between 2.5 and 5. In practical applications, fsamp>>fscan, typically by a factor between 500 and 2000.
In the OCT system according to FIG. 2, the interference signal 34 is therefore sampled in constant time intervals that are defined by the reciprocal value of fsamp. In the image generation that is based on a Fourier transformation, however, it is required that the sampling points are equidistant in the optical frequency. In sampling that is equidistant in time, this is only the case if the optical frequency change of the tunable light source 12 is linear in time during the wavelength sweep, but this can hardly be achieved in practical applications. This is the reason why the digital data 42 usually is reprocessed or “resampled.” In a resampling process, the interference signal 34 is determined at certain supporting points in time by interpolating the measured digital data 42, wherein the supporting points are chosen such that the sampling points are equidistant in the optical frequency of the interference signal 34.
Alternatively, the detection clock signal 38 may also be generated in the form of a time-variable clock signal realized in such a way that the samples are already generated linearly in the optical frequency differential. Such a clock signal is known as the “k-clock” of OCT systems, wherein the “k” stands for the wave number. In this case, the k-clock signal typically is a pulse train with variable intervals in the time domain, but identical intervals in the optical frequency domain. An OCT system with such a k-clock is schematically illustrated in FIG. 3. The k-clock is generated directly from the light of the tunable light source 12 by an additional interferometer 44 with constant arm length differential. The k-clock may be defined, for example, by the zero crossings of the interference signal in the interferometer 44. In this way, a non-linear progression of the wavelength sweep of the light source 12 is automatically compensated.
It is generally attempted to increase the speed of the depth scans. Since a depth scan corresponds to one wavelength sweep in swept-source OCT, this means that the frequency fsweep needs to be increased. For this purpose, the present inventors have proposed a tunable Fabry-Perot filter that allows sweep frequencies of several hundred kHz up into the MHz range. When using such a Fabry-Perot filter, the wavelength is typically modulated approximately sinusoidally, but the portion in which the wavelength changes at least approximately linearly with time is preferably used for the wavelength sweep. In order to avoid delay times, the prior art already utilizes so-called buffered tunable light sources, in which only a largely linear section of the entire wavelength sweep is used and then optically split into a plurality (e.g. 2, 4 or 8) of wavelength sweeps that are mutually delayed in time and reassembled. In this way, a continuous sweep signal is generated, in which the wavelength changes with time in a ramp-like or sawtooth-like fashion. The sweep rate can also be effectively increased with the number of buffered wavelength sweeps.
However, the inventors have discovered that problems with the quality of OCT images can arise as the sweep rate increases. This is the case, in particular, if each depth scan is not triggered individually as described with reference to FIG. 2, but a complete data frame, which is composed of the detection data of a plurality of wavelength sweeps, rather is generated in the form of a continuous data stream with a single trigger. In this case, a data frame may comprise a complete line of depth scans such as, for example, around 1000 depth scans that jointly form a two-dimensional image. This option can be used with higher sweep rates fsweep and buffered light sources, i.e. those without delay times between sweeps, because problems caused by trigger delay times, which occur if each depth scan would be triggered individually, can be avoided. Such trigger delay times occur in typical data acquisition cards between the last sample of a just triggered acquisition and the focusing on the next trigger.
The following concrete problems with the image quality arise in practical applications and are explained below with reference to FIGS. 4 and 5.
FIG. 4 shows a 2D frame consisting of 1600 depth scans on the nail bed of a human fingernail. The entire frame was recorded with one continuous data stream, i.e. in response to a single “common” trigger that is also referred to as “frame trigger” in the present disclosure. FIG. 4 shows that a certain blur develops in the image from the left side toward the right side.
FIG. 5 likewise shows a 2D frame that was recorded with a single frame trigger. In this case, FIG. 5 shows a 2D depth scan of the skin of a human finger. Although the image quality is adequate with respect to the actually interesting structures, interfering horizontal lines occur in the image.
Although the above-described tomographic method for producing depth scans in a sample is the most commonly used type of OCT, the basic principle of OCT can also be used for other purposes. One such example is a photoacoustic detection system of the type described in the article “Intrasweep Phase-Sensitive Optical Coherence Tomography For Noncontact Optical Photoacoustic Imaging” (Cedric Blatter, Branislav Grajciar, Pu Zou, Wolgang Wieser, Aart-Jan Verhoef, Robert Huber and Reiner A. Leitgeb; Optics Letters, Vol. 37, 21st Edition, pp. 4368-4370 (2012)), wherein this article is hereby incorporated into the present disclosure by reference. In simplified terms, a shockwave is triggered in a sample such as, for example, in a tissue with a light pulse in such a photoacoustic detection. Similar to ultrasound methods, this shockwave could then be detected with corresponding “microphones.” However, it is also possible to use OCT for the detection of the shockwave, namely by detecting the motion of the sample surface—i.e. the result of the shockwave in the issue—with the aid of OCT. The detection set-up is similar to conventional OCT systems for depth scans, but the recorded data is evaluated differently because the motion of the surface is so fast that several oscillations occur per frequency sweep. In contrast to the conventional version of OCT, only the surface, particularly the motion of the surface, is detected in this case, but no depth information.